The offshore production of oil and gas requires the use of fluid-carrying systems to convey production fluids from a subsea well to the water surface. Such fluid-carrying systems typically include a rigid, substantially vertical conductor pipe which is cemented at its lower end to the subsea well. The upper end of the conductor pipe is connected to a wellhead which is typically located on the deck of an offshore production platform. Tubing, which is located inside the conductor pipe, conveys production fluids from the subsea well to the wellhead. At the upper end of the wellhead, valves in a "Christmas Tree" are manipulated to regulate the pressure and flow rate of the production fluids. A flowline conveys production fluids from the Christmas Tree to a manifold on the deck of the offshore platform. The manifold routes the production fluids to treating and separation equipment which processes the production fluids. The flowline is rigidly anchored to the deck of the offshore platform or is rigidly connected to the manifold. Such rigid connection completes the fluid-carrying system between the subsea well and the offshore platform.
While rigid offshore production platforms are typically used to support a fluid-carrying system in water less than 1000 feet deep, compliant offshore platforms such as guyed towers or tension leg platforms have been designed for greater water depths. Such compliant offshore platforms are less massive than are rigid offshore platforms and "comply" with loading forces induced by wind, waves, and ocean currents. When acted upon by such forces, a compliant offshore platform will be displaced from its equilibrium position above a subsea well. Such displacement may be vertical as well as horizontal. As the displacing force subsides, the compliant offshore platform will return to its equilibrium position above the subsea well.
A fluid-carrying system supported by a compliant offshore platform must be sufficiently flexible to compensate for movement of the compliant offshore platform from its equilibrium position. Fluid-carrying systems typically compensate for such movement by using a flexible flowline, such as an elastomeric hose, between the wellhead and the deck of the offshore platform. However, a flexible flowline manufactured from an elastomeric hose is subject to certain limitations. For example, "sour" production fluids frequently contain chemical compounds such as hydrogen sulfide and carbon dioxide which deteriorate the materials used in an elastomeric hose. Thus, an elastomeric hose carrying sour production fluids must be periodically replaced. In addition, the pressure of the production fluids may exceed 5000 psi. An elastomeric hose must be reinforced in order to handle such fluid pressures without failing. Such reinforcement reduces the flexibility of an elastomeric hose and correspondingly increases the minimum bending radius of the hose. An elastomeric hose with a greater bending radius will therefore require more deck space than will a hose with a lesser bending radius. Consequently, the use of elastomeric hoses will reduce the number of wells that can be produced by an offshore platform.
In addition to elastomeric hoses, slip joints have been used in a fluid-carrying system to compensate for movement of a compliant offshore platform from its equilibrium position. Such joints typically have an inner pipe which slidably moves within an outer pipe. The annulus between the inner and outer pipe is sealed with elastomeric seals to prevent production fluid from leaking into the ambiance. However, slip joints are undesirable in a fluid-carrying system between a wellhead and an offshore platform. Slip joints sized to accommodate the movement of an offshore platform are long and therefore require a great deal of vertical space between the decks of the offshore platform. Such space is typically limited by the dimensions of the offshore platform. Additionally, slip joints in a fluid-carrying system are undesirable when conveying a sour production fluid. The elastomeric seals used in such slip joints are subject to deterioration induced by compounds in the sour production fluid and will leak. Furthermore, the movement of the inner and outer pipes of the slip joint will abrade the elastomeric seal as the inner pipe reciprocates within the outer pipe. Such abrasion reduces sealing effectiveness as the elastomeric seals become worn. Finally, a slip joint is limited because it moves linearly and does not accommodate lateral movement of the offshore platform about the subsea well. This lateral movement of the offshore platform can be so severe as to damage the slip joint.
To avoid certain limitations of flexible hoses and slip joints, various combinations of in-line swivels and concentric swivels have been used to accommodate relative movement in a fluid-carrying system. However, swivels rely on elastomeric seals to seal the moving elements of the swivel. Such seals are subject to deterioration induced by a sour production fluid as previously described. Furthermore, swivel connections are limited to a particular range of movement and do not flex beyond such operating range. If a severe storm should displace an offshore platform to an extraordinary distance from its equilibrium position, the pipe connecting the swivels in a fluid-carrying system would not plastically deform but would rupture. Such rupture would release the pressurized production fluid into the ambiance.
A need, therefore, exists for a flexible apparatus in a fluid-carrying system to accommodate movement of a compliant offshore structure about a subsea well. Such flexible apparatus should be capable of conveying a sour production fluid which is produced at high pressures. The flexible apparatus should also accommodate, without leakage, cyclic as well as extreme displacements of a compliant offshore platform from its equilibrium position.